The Rayburn House Office Building was locked down for more than four hours today after a New Jersey congressman reported hearing the sound of gunfire in the building's garage.

The noise apparently came from construction work being done in the building, Capitol Police spokeswoman Sgt. Kimberly Schneider said.

"There were some workers . . . in the area of the Rayburn garage, in the elevator area, and in doing their routine duties they made some sort of a noise that sounded like shots fired," Schneider said.

She said that while the police investigation is continuing, the construction work is a "plausible explanation" for the noise that also briefly closed the Capitol building and locked down D.C. public schools.

"This is very good news . . . It's a very happy conclusion to a long day," Schenider said at her third news conference of the afternoon. All three were carried live by cable news networks. The Rayburn building was locked down shortly after 10:30 a.m. after Capitol Police received a call from the office of Rep. Jim Saxton (R-N.J.). Saxton asked one of his staffers to call police after hearing what he thought were gunshots, a member of his staff said late this afternoon.

The building was reopened about 3 p.m.

In between, House members, congressional staff members and visitors to the sprawling building on Independence Avenue were told not to leave their offices as SWAT officers -- dressed in black, guns at the ready -- made a methodical, door to door sweep. The 2.4-million-square-foot Rayburn building has four floors above ground, two basements, and three levels of underground garage space.

Schenider said early this afternoon that police had to insure that "every single person who is in the Rayburn building belongs in the Rayburn building."

"That means doing it the old fashioned way," she said as the search for suspects and weapons continued. "We're going door to door, floor by floor. Every inch -- every square inch -- of the Rayburn building is going to be cleared out today."

"We are handling this as a very serious matter," Schneider told reporters. "I want to make that clear."

The Capitol building itself, connected by a tunnel to the Rayburn building, was largely unaffected. At first, no one was allowed to leave the Capitol. Later, it was open for above-ground access, although its tunnels remained closed, the House Emergency Communications Center said in an e-mail alert at 11:27 a.m. Still later, the Capitol was locked down briefly, then reopened.

Meanwhile, all public schools in the District briefly went into lockdown as a result of the situation around the Capitol, a spokeswoman for the school system said. School officials wanted to err on the side of caution, she said. The schools were reopened by mid-afternoon.

Schneider said the single caller's report at 10:30 a.m. was "sufficient enough" reason to lock down the building.

The sound of gunfire was heard on the third level of the three-level Rayburn garage, police said. There's a House gymnasium on the second level and a practice firing range -- used by Capitol Police -- on the first level of the garage.

At 10:45 a.m., Capitol Police sent an e-mail alert with instructions for people inside the Rayburn building: "If you are in the [building] . . . then Shelter in Place. Quickly move into the nearest interior office space or interior hallway and away from windows. The Capitol Police are investigating reports of gunfire in the Rayburn HOB."

The alert added: "Remain calm. Await further instructions. Do not leave the building."

A staffer of Rep. Jack Kingston (R-Ga.) was in the House gym when the Rayburn building was locked down. She was taken to a hospital by ambulance "just a little shaken up under the circumstances," the congressman's spokesman, David All, reported. She was released from the hospital before 2 p.m.

Meanwhile, an aide to Rep. David Price (D-N.C.), whose office also is in Rayburn, said Capitol Police suddenly were "bowling through the hallways," shouting for people to get into their offices, shut their doors, and stay there.

About a half dozen workers inside the second-floor Rayburn offices of U.S. Rep. Jim Moran (D-Va.) were continuing to answer calls and carry on with their work as usual, legislative assistant Heath Bumgardner said this morning.

"People were concerned but we go through a lot of alarms here, some real, some false," Bumgardner said.

The Senate continued to meet after the initial reports of gunfire. The House was not in session, but one committee was meeting.

Rep. Peter Hoekstra (R-Mich.), conducting a morning House Intelligence Committee hearing on "the media's role and responsibilities in leaks of classified information," interrupted a witness to tell people in the room they should not leave and that the doors were being closed.

"It's a little unsettling to get a Blackberry message put in front of you that says there's gunfire in the building," he said.

About 60 people, including Capitol Hill staffers, media representatives and about 10 members of Congress, were confined to the hearing room, where they passed time chatting on cell phones, checking their Blackberries, perusing newspapers and sharing a single bathroom.

Some members of Congress were escorted out of the room after an hour. But Hoekstra stayed.

"I have no confirmation of gunfire. We have confirmation of a loud noise," he said at one point, as people inside the building began questioning whether gunshots were ever really fired.

Finally, at about 2:45 p.m., SWAT officers entered the hearing room.

"Hands on your head," they said, and everyone did just that.

Then, one officer asked: "Is there anybody in this room that doesn't belong here?"

No identification cards were checked. But within minutes the people in the room were escorted into the hallway, then -- after passing through a metal detector -- out onto the street.

For people outside the building, congressman Kingston's office provided Internet updates on the situation though "Jack's Blog."

Sample entries:

12:19 p.m.: "The SWAT team just moved in on the House gym which is about four five floors below our office."

1:39 p.m.: "The Capitol Police just checked out our office. The officers were very courteous and respectful. We're glad they're doing such a thorough job. All is quiet."

2:59 p.m.: (in response to a question from a reader of the blog): "Yes, all Member offices have a restroom. And we also have Georgia Peanuts, Vidalia Onions, and Coca-Cola's in case we're here all night."

Capitol Police spokeswoman Schneider said law enforcement officials did the right thing, shutting down the building.

The investigation, she said, was prompted by a "valid call" to police.